I was using Workflowy extensively but realized that navigating that huge document was painful, so I switched to OmniOutliner which has a navigation panel that lets me jump to the section I needed to work on, extensive key commands and formatting.

I'm still looking for the tool that will allow me to see the forest and the trees (on drilldown) - it'll probably look like a mashup of OmniFocus, OmniOutliner, Omnigraffle, Evernote and Numbers. Perhaps I'm just pining for the promise of OpenDoc.